TVS protection for CAN bus

Question

It looks like every chip manufacturer has their own CAN bus TVS product nowadays, e.g. ON Semi NUP2105, Semtech 2492SQ, Bourns T24CAN, Nex PESD2CAN and many others.

Common for all of them are surprisingly high stand-off and breakdown voltages, typically around 24 and 30 V. These seem to be inline with ISO 11898 max bus voltage specification for 24 V systems; way too much for 12 V systems.

Furthermore, most transceivers are designed to work off a 5 V supply and 3.3 V devices becoming more common. There is no way these drivers can put up more than 5 V on the line, and even if we add generous common-mode tolerance it will still be nowhere close to TVS specs.

While doing my research I've stumbled upon this interesting board from TI, demonstrating various ESD protection circuits. All of them use SM712 diodes, designed for RS-485. They have much lower and asymmetrical breakdown specs -7..12 V, which seems to be ideally suited for CAN.

So, my question would be this: why do manufacturers keep churning out those high-voltage devices, and why are devices like the SM712 nowhere to be seen except on those obscure TI boards?

Update:

While most comments and answers are focused on automotive applications and applicable ISO standards, one is standing out. As @Lundin pointed out: "TVS value should be picked after the voltage levels on the electronics you wish to save, no after the expected level of the spikes". This seems to be obvious, but somehow missed by many.

So, I decided to dig in datasheets, and see what is actually out there. On digikey there are 1000+ CAN transceivers, but only of them 300+ are automotive. This seemed promising to support my point. Of course, most of the automotive parts are tolerant to -27V...+40V spikes, some of them going as high as +400V (short pulses).

However non-automotive parts were big surprise. While some (like MAX305x series) do not allow more than 12.5V, a lot of chips can survive 36..40V on a bus (both differential and common mode).

So, I guess both @Lundin's comments taken together do answer my question: There is not much need for lover voltage TVS protection chips because majority of CAN transceivers can tolerate much higher spikes than operational bus voltage.

Answer 1

The reason why CAN protection devices have a stand-off voltage above 24 V is not linked to the CAN bus itself. You are right: most of the CAN transceivers work below 5 V.

But in an automotive environment, CAN protection must be compliant with many standards simulating incidents/mistakes that can occur in a car.

In particular, a car must be robust in case of a “jump start” (ISO 16750 standard). This test consists in applying 24 V on the CAN bus to simulate for example:

• A wrong connection of an auxiliary battery in series with a flat battery of a passenger car;
• A garage battery booster with a wrong voltage selection (24 V instead of 12 V) connected to power a passenger car with no battery;
• A truck battery (nominal voltage 24 V) connected to power a passenger car to start the engine.

And certainly, others I cannot imagine.

The test duration is 60s +/- 10%. The normative constraints are described in the doc below:

CAN-bus-protection-ST-ESDCAN-series presentation

A CAN TVS with a 5 V breakdown voltage would enter in avalanche mode with an unlimited current and would burn well before the end of the 60s of test duration.

So the principle for the TVS is to avoid entering conduction mode, and therefore, to increase the breakdown voltage to above 24 V.

That’s a compromise with the clamping voltage, but most of the automotive CAN transceivers have absolute maximum ratings (AMR) above 24 V. However, these transceivers cannot pass ESD and ISO 7637 transients alone without the help of a CAN TVS.

You may see that some of the CAN TVS protection devices have stand-off voltages of up to 35 V or 36 V. These products are intended for commercial vehicles (trucks, off-roads, …) with 24 V batteries, like ESDCAN05 or ESDCAN06 for example.

If your application is not linked to automotive, then you may find TVS with a better compromise as all these automotive standards do not apply. But it depends on your CAN protocol implementation. Indeed, the voltages tolerances are quite wide.

Keep in mind that the TVS in your circuit must be as close as possible to the connector (better to stop the transients at the entrance) and the stand-off voltage selection must be as close as possible to the operating voltage of your CAN bus including the tolerances.

For example, if your CAN bus operates between -0.3 V and 5.5 V maximum, then choose a unidirectional TVS diode with a stand off voltage up to 5.5 V. Do not select a bidirectional 5.5V diode that will clamp negative transient voltages below -5.5 V only! The unidirectional will clamp negative transient voltages below -0.6 or -0.8 V. It is much safer.

I hope this clarifies things.

Answer 2

These seem to be inline with ISO 11898 max bus voltage specification for 24V systems. Way too much for 12V systems.

According the datasheet of the NUP2105L, the maximum bus voltage for a 12V system can be up to 25V volts.